1. Field Of The Invention
This invention relates to computer systems and, more particularly, to methods and apparatus for switching high voltages in circuitry manufactured by processes typically used to provide low voltage tolerant integrated circuitry.
2. History Of The Prior Art
A major trend in the manufacture of personal computers is toward portable computers which are able to provide most of the abilities of desktop computers but are packaged in very small and light packages. In order to accomplish this, such portable computers must provide the large storage capacity and the processor speed necessary to run large programs. A typical prior art portable computer uses a great deal of power in order to meet these requirement.
At the same time, computer designers are modifying the components used in such portable computers so that the computers are able to run for long periods while consuming very small amounts of power. In order to reduce power consumption and extend battery life, much of the integrated circuitry used in portable computers is being redesigned to run at low voltage levels. This reduces the power usage and allows more components to be placed closer to one another in the circuitry. The modern circuitry and components used in portable computers are being designed to operate at 3.3 volt levels. This helps a great deal to reduce the power needs of portable computers. Part of this redesign reduces the thickness and size of the various layers, regions, and conductors utilized in order to reduce the size and power consumption of the various components used in the integrated circuits
However, at the same time, the desire to make portable computers offer all of the advantages of desktop computers opposes this salutary result. Many of the advantages offered by desktop computers require higher voltages to function. For example, one real convenience is the ability to change the BIOS processes as improvements in the computer or its peripherals occur. Historically, this has been accomplished by removing the EPROM or similar circuitry providing the read only memory for storing the BIOS processes and replacing it with new circuitry at additional cost. This is a complicated operation beyond the abilities of many computer users. Recently, flash electrically-erasable programmable read only memory (EEPROM memory) has been used to store BIOS processes. This memory may be reprogrammed without removing the BIOS circuitry from the computer by running a small update program to reprogram the BIOS circuitry when the BIOS processes change. However, reprogramming flash EEPROM memory requires from seven to twelve volts to accomplish. The integrated circuits designed for low voltage use are typically not able to tolerate such high voltages without physical damage.
Another form of flash EEPROM memory array provides another example of high voltage requirements in portable computers. Recently, a new form of long term random access storage has been devised using flash EEPROM memory arrays. An example of a flash EEPROM memory array which may be used in place of a hard disk drive is given in U.S. patent application Ser. No. 07/969,131, entitled A Method and Circuitry For A Solid State Memory Disk, S. Wells, filed Oct. 31, 1992, and assigned to the assignee of the present invention. These arrays provide a smaller lighter functional equivalent of a hard disk drive which operates more rapidly and is not as sensitive to physical damage. Such memory arrays are especially useful in portable computers where space is at a premium and weight is extremely important. However, these flash EEPROM memory arrays also require much higher voltages for writing and erasing data than can be tolerated by prior art integrated circuitry adapted to operate at low voltages. Consequently, portable computers using low voltage circuitry have not been able to utilize these modern advantages.
Recently various circuitry has been devised to allow certain high power operations to be accomplished within integrated circuits designed for processes typically used for low power circuitry. For example, voltage pump circuitry has been designed for providing higher 10 and 12 volt levels in semiconductor circuitry designed for a 3.3 volt process. One of the problems discovered in attempting to utilize such circuitry is that it is difficult to discharge nodes raised to higher voltage levels because of the lower levels of voltage and current which the circuitry designed for a low voltage process will tolerate.